Latent hypoxia

[1] The minimum tissue and vascular partial pressure of oxygen which will maintain consciousness is about 20 millimetres of mercury (27 mbar).

This equates to a minimum arterial oxygen partial pressure (PaO2) of 29 millimetres of mercury (39 mbar) at 868 ml/min cerebral flow.

A critical PO2 of 30 millimetres of mercury (40 mbar) in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive.

[9] The laryngospasm will eventually relax, and if the diver is still underwater then water will enter the airway and may reach the lungs which will cause complications if resuscitation is successful, and secondary drowning is possible.

[9] The time between loss of consciousness and death varies considerably depending on a number of factors but can be as little as two and a half minutes.

[10] If the diver's airway is protected by a full-face mask or diving helmet, the immediate risk is death by asphyxiation, which can occur within a few minutes of cessation of breathing.

The diver who has lost consciousness due to latent hypoxia is already hypoxic, and should be brought to the surface as soon as possible, while protecting the airway.

There are no contraindications to immediate surfacing for a breath-hold diver, and the mouth and nose may be blocked to prevent involuntary aspiration of water.